Spectacle lenses are generally produced from blanks via material-removing machining of the prescription surfaces within the context of prescription fabrication. After this machining step, the optical properties of the lens are defined by the surface form produced in this way. The machined surface is then polished further, by which means a microscopically smooth surface and the desired optical properties of the lens are intended to be achieved.
For the purpose of polishing, use is generally made of a polishing tool, of which the polishing surface formed by a polishing agent carrier is matched approximately to the form of the surface of the lens that is to be polished. This at least approximate matching to the form of the lens surface to be polished can be handled with tolerable outlay for the polishing of spherical or toroidal prescription surfaces. However, in the case of spectacle lenses the proportion of highly accurate free-form surfaces increases sharply, normally being generated with the aid of diamond tools on CNC-controlled machines in the rotary process.
Aspherical or point-symmetrical surfaces and free-form surfaces have curvatures which change over the surface. The polishing tool is moved at least over a part of this irregularly curved surface during the polishing of such free-form surfaces. The polishing tool must therefore be able to match the respective local curvature with its flexural rigidity and elasticity, specifically in such a way that the polishing pressure is as constant as possible over the contact area. Only then is the result a determinable constant removal and the polished surface is polished uniformly. If this is not ensured, the surface and the topography of the free-form surface will be deformed and its optical quality impaired. On the other hand, local irregularities which have arisen on account of the material-removing machining process, for example grooves, waves or central defects, should be eliminated without a trace.